The life cycle of retroviruses involves a stage when the virus' genetic material is inserted into the genome of a host cell. This step is essential because the inserted viral nucleic acid, the provirus, is replicated through the host cell machinery.
As part of this process the RNA genome of the retrovirus is replicated through a double-stranded DNA intermediate prior to insertion into the genome of the host cell. The initial conversion of the viral RNA molecule into a double-stranded DNA (dsDNA) molecule is performed by a reverse-transcriptase. The dsDNA is then integrated into the hose cell genome by an integrase to further be replicated by the host cellular machinery. The reverse transcriptase and the integrase required for the conversion of the RNA into dsDNA and for the integration into the host genome are carried within the viral particle during infection. The proviral DNA is finally transcribed using the host machinery into multiple RNA copies. These RNA molecules are then translated into viral peptides or proteins or integrated into viral particles which are released from the cell into the medium or extracellular milieu.
A retroviral RNA genome usually comprises 6 typical regions leading to the expression of multiple proteins. These region include the gag, pol and env gene sequences associated with a packaging signal, a psi (ψ) signal and flanked by 5′ and/or 3′ long terminal repeats (LTR) regions. The gag gene leads to the expression of the protein components of the nucleoprotein core of the virus, while the pol gene products are involved in the synthesis polynucleotides and recombination. The env gene codes for the envelope components of the retrovirus particle. 5′ and 3′ LTR regions include promoters and assist in the integration of the viral genome into the chromosomal DNA of the host cell. The psi signal refers to the retroviral packaging signal that controls the efficient packaging of the RNA into the viral particle.
Because of their ability to form proviruses, retroviruses are useful to modify the genome of a target or host cell and various modifications have been made to retroviruses for use in gene therapy. Gene therapy using retroviral vectors is generally performed by adding a heterologous polynucleotide to the viral genome which encodes or produces a polypeptide or transcript of interest, packaging the recombinant genome into a viral particle and infecting a target host cell. The target cell will then incorporate the exogenous gene as being a part of a provirus.
Most retroviral vectors have been rendered “defective” to avoid uncontrolled spread and production of virions. However, little is reported about the development of replication competent retroviral vector systems.